1. Field of the Invention
The present invention relates to a stencil printing machine, especially to a machine having a mechanism for preventing ink-leakage from a printing drum.
2. Description of the Related Art
In Japanese Patent Application 63-28553 (Japanese Laid Open Publication No. 1-204781), Japanese Patent Application 1-47029 (Japanese Laid Open Publication No. 2-225078), and Japanese Patent Application 2-223550 (Japanese Laid Open Publication No. 4-105984), the present applicant proposed a stencil printing machine. This stencil printing machine has a printing drum, an ink-applying roller situated inside the printing drum, and a back-pressing roller (an opposing drum) disposed under the printing drum. The printing drum has a base body comprising two annular members coaxially arranged at a predetermined distance therebetween and a connecting member connecting the two annular members. A flexible and ink-permeable circumferential wall is wrapped around the base body. Namely, one end of the circumferential wall is fixed to the base body, and the other end is attached to the base body by an elastic member. The ink-applying roller is situated inside the printing drum for providing ink to an inner surface of the circumferential wall. The ink-applying roller is movable between a pressing position for pressing the circumferential wall to be deformed radially outwardly and a standing position for releasing the circumferential wall from deformation. The back-pressing roller is situated under the printing drum and arranged parallel with the printing drum. The back-pressing roller is driven to rotate in a direction opposite to that of the printing drum. In the constitution explained above, a perforated stencil sheet is wrapped around an outer circumferential surface of the circumferential wall of the printing drum, and then stencil printing is started. A printing sheet is supplied between the printing drum and the back-pressing roller. The ink-applying roller deforms the printing drum outwardly. The printing sheet is conveyed while being sandwiched between the printing drum and the back-pressing roller, thereby being printed.
Further, in Japanese Patent Application No. 3-162218 (Japanese Laid Open Publication No. 4-361043), the present applicant proposed a stencil printing method using the stencil printing machine of the constitution stated above. In this method, the back-pressing roller has a clamp for holding a leading end of the printing sheet. The printing sheet held by the clamp on the back-pressing roller is peeled off from the printing drum just after printing, so that a distinct and uniform printed image without faintness can be obtained and set-off does not occur.
Further, in Japanese Patent Application No. 5-306033 (Japanese Laid Open Publication No. 7-137419), the present applicant proposed a stencil printing machine in which a longitudinal position, i.e. head and tail position, of a printed image on a printing sheet can be adjusted by adjusting a rotational position of a printing drum relative to a back-pressing roller.
Further, in the case where a clamp is disposed on the back-pressing roller for holding a leading end of the printing sheet, the back-pressing roller must have a recess formed therein for avoiding interference with a rear edge of the clamp, so that the clamp can open and close. However, when the clamp of the back-pressing roller meets the printing drum, ink may leak from between the printing drum and a stencil sheet, and transfer into the recess formed adjacent to the rear edge of the clamp. And, in Japanese Patent Application No. 7-214075 (Japanese Laid Open Publication No. 9-39359), the present applicant proposed that a strip of an elastic sheet be attached to an outer circumferential surface of the back-pressing roller parallel to a generating line for covering the clamp to prevent such a trouble.
FIG. 18 is a schematic view illustrating a publicly known basic constitution of a stencil printing machine. This machine comprises the printing drum, the ink-applying roller, the back-pressing roller, each of them are explained above, and the clamp disposed on the back-pressing roller for holding the leading end of the printing sheet. In this drawing, the reference numeral "10" indicates the printing drum, and "14" indicates the back-pressing roller. The printing drum 10 has a frame. The frame is composed of a pair of annular members 16, 16 and a connecting member 18 connecting the pair of annular members. The annular members are arranged at both end portions of an axis of the printing drum. The connecting member is parallel to the axis of the printing drum. A flexible porous sheet 20 is in a developed form of rectangular, and the sheet is wound around the frame. Namely, a front edge 20a and a rear edge 20b are attached to the connecting member in such a manner that both side edges of the sheet contact outer circumferential surfaces of the annular members. Especially as concerns attachment of the rear edge 20b to the connecting member 18, Japanese Patent Application 1-47029 (Japanese Laid Open Publication No. 2-225078), Japanese Patent Application 5-306028 (Japanese Laid Open Publication No. 7-137415), and Japanese Patent Application 5-306029 (Japanese Laid Open Publication No. 7-137416) should be referred to.
A circumferential wall of the printing drum 10 is composed of the flexible porous sheet 20, and a stencil sheet 19 is wrapped around an outer surface of the wall with its leading end held on the connecting member 18 by a clamp 21.
The front edge 20a and the rear edge 20b of the flexible porous sheet 20 are of non-porous structure. A middle portion 20c between the edges is of porous structure allowing ink to pass through. The non-porous edges 20a and 20b of the flexible porous sheet 20 and the connecting member 18 constitute a stencil-sheet leading-end attachment portion 10a. The portion is a non-porous strip-shaped area elongated along a generating line of the printing drum 10 between both side edges of the drum.
On the back-pressing roller 14, a traverse groove 22 is formed parallel to its center axis along a generating line. The printing drum 10 and the back-pressing roller 14 have the same diameter. The drum and the roller are driven to rotate in directions opposite to each other in such a manner that the stencil-sheet leading-end attachment portion 10a and the traverse groove 22 of the back-pressing roller periodically meet in synchronization with each other. As shown in FIG. 18, the rotating direction of the printing drum 10 is anti-clockwise, and the rotating direction of the back-pressing roller 14 is clockwise.
An ink supplying roller 12 is situated inside the printing drum 10. The ink supplying roller 12 is driven to rotate by an axis 13 with its outer circumferential surface contacted with an inner circumferential surface of the printing drum 10. A cam 24 is attached to the annular member 16 in the vicinity of the connecting member 18. An annular cam-follower 23 provided on the axis 13 engages with the cam 24. Thus, when the ink supplying roller 12 passes the connecting member 18 during the drum rotation, the roller 12 is lifted up from the inner circumferential surface of the printing drum 10 so that collisional contact of the roller with the connecting member can be avoided.
On the back-pressing roller 14, a clamp 25 is attached to a rear portion of the traverse groove 22 relative to the rotating direction of the back-pressing roller. The clamp 25 holds a leading end of a printing sheet on the back-pressing roller 14. The printing sheet is attached to the back-pressing roller 14 with its leading end held by the. clamp 25. The printing sheet is inserted between the back-pressing roller 14 and the printing drum 10 during the clockwise rotation of the back-pressing roller 14 as shown in the drawing. Ink is provided to the inside of the flexible porous sheet 20 by the ink supplying roller 12, and passes through holes formed in the ink supplying roller 12, thereby transferring to the printing sheet through perforations of the stencil sheet 19 to form an image thereon.
In Japanese Patent Application 5-306033 (Japanese Laid Open Publication No. 7-137419), there disclosed such a stencil printing machine that incorporates a head-and-tail adjusting device for adjusting a position of an printed image on a printing sheet. In such stencil printing machine, when a rotational phase of the printing drum 10 relative to the back-pressing roller 14 is in a standard condition, a border line 26 dividing the porous structure 20c and the non-porous front edge 20a of the flexible porous sheet corresponds to a line 28 on the outer circumferential surface of the back-pressing roller 14. Similarly, a borderline 32 dividing the porous structure 20c and the rear edge 20b of the flexible porous sheet corresponds to a line 34 on the outer circumferential surface of the back-pressing roller 14. When a rotational phase of the printing drum 10 relative to the back-pressing roller 14 is changed for head and tail adjustment, the lines 28 and 34 move in the directions shown by arrows 38 and 40, respectively, and the porous structure 20c may meet the clamp 25.
As illustrated in FIG. 19, rather large recess 27 is formed in the rear side of the clamp 25, so that the clamp 25 can lean in pivotally moving around an axis 62. Then, suppose that the porous structure 20c of the flexible porous sheet meets the clamp 25, as stated above. Ink is pressed by the ink supplying roller 12 against a part of an inner surface of the porous structure 20c corresponding to the recess 27. Ink is squeezed out into the inside of the stencil sheet 19 through holes of the porous structure 20c, but the ink is not fully supported by the stencil sheet on the outside. Accordingly, as illustrated in FIG. 19, a part of the stencil sheet 19 corresponding to the recess 27 swells locally. Once ink is squeezed out in this way, since it has high viscosity, all of the squeezed ink cannot return to the inside of the porous structure 20c even after releasing of the ink-pressing force. Thus, if such phenomenon is repeated during rotation of the printing drum, ink flows across the porous structure 20c from the inside to the outside of the printing drum, thereby accumulating between the outer circumferential surface of the printing drum and the stencil sheet. And, it is clear from FIG. 19 that such ink-leakage due to shortage of back-pressing at the recess 27 similarly occurs in a stencil printing machine having a rigid porous structure. Accumulation of the ink leads to ink-leakage at both axial end portions of the printing drum. Further accumulation of the ink may force the swelled stencil sheet to be strongly pressed against the clamp 25 or a corner of the recess 27, thereby causing damage to the sheet.
An invention shown in FIG. 20 has been proposed by the present applicant in Japanese Patent Application 7-214075 (Japanese Laid Open Publication No. 9-39359) in order to settle such problem. The drawing shows a strip-shaped elastic sheet 52 situated on the outer circumferential surface of the back-pressing roller 14 along a generating line for covering the clamp 25.
The problem is settled to some extent by the strip-shaped elastic sheet 52. However, since the elastic sheet 52 easily bend, if the machine operates for a long time while the porous structure 20c of the flexible porous sheet repeatedly contacts the clamp 25, ink-leakage due to deformation of the elastic sheet 52 may occur through the ink-leakage process explained in referring to FIG. 19.
Then, the present applicant improved the stencil printing machine and proposed a new one as shown in FIGS. 10 to 17 in Japanese Patent Application 9-183113 to further reliably prevent the ink-leakage by the leakage process explained above.
As illustrated in FIG. 10 and FIG. 16, the back-pressing roller 14 is driven to rotate around an axis 68. The axis 68 is attached to a predetermined position of a non-illustrated frame. The back-pressing roller 14 is driven to rotate around the axis 68.
Plural clamps 25 are disposed on the back-pressing roller 14. The back-pressing roller 14 has the axis 62. The plural clamps 25 are fixed to the axis 62 at a predetermined distance therebetween. One end of the arm 101 is fixed to one end portion of the axis 62. A driven member 102 is attached to the other end of the arm 101. A cam 100 is adjacent to one end of the back-pressing roller 14 and fixed to the not-shown frame of the back-pressing roller 14. The arm 101, the driven member 102, and the cam 100 are arranged on the distant end side in the perspective view of FIG. 16.
The driven member 102 engages with the cam 100. The cam 100 is fixed to the frame. The driven member 102 follows the cam 100 while rotating with the back-pressing roller 14. The outer shape of the cam 100 enables the clamp 25 to open or close at appropriate timing in synchronization with rotation of the back-pressing roller 14.
An arm 70 in the shape of L is rotatably connected to the axis 68 at an elbow-portion thereof. One end of the arm 70 is connected to a link 84. The link 84 is connected to one end of the axis 62 via a link 64. The other end of the axis 62 is connected to the arm 101. A clamp cover 66 is rotatably connected to the other end of the arm 70 via a connecting axis 74. The clamp cover 66 is a member that covers gaps between the clamps 25 arranged at intervals and the recess 27 formed in the back-pressing roller 14 at a rear end side of the clamps 25. As illustrated in FIG. 16, a bracket 80 is fixed to an end surface of the back-pressing roller 14. A link 78 is rotatably connected to the bracket 80 via a pin 82. A pin 76 is fixed to an end portion of the clamp cover 66. The pin 76 is rotatably connected to the link 78 while passing through a groove 103 formed in the other end of the arm 70.
Operation in the constitution explained above will be explained. The back-pressing roller 14 is driven to rotate. The cam 100 remains static since it is fixed to the frame side. The driven member 102 connected to the back-pressing member 14 follows the shape of the cam 100 while rotating around the axis 68. The movement of the driven member 102 causes the axis 62 to rotate, thereby opening or closing the clamp 25. The rotation of the axis 62 causes arm 70 to rotate, and the clamp cover 66 connected to the arm 70 moves while being regulated by the link 78.
In a predetermined timing at which a printing sheet is supplied between the back-pressing roller 14 and the printing drum, the driven member 102 starts to contact and follow the cam 100, as illustrated in FIG. 11. The axis 62 rotates, thereby causing the clamp 25 to open. Simultaneously with this movement, the clamp cover 66 is slightly lifted up while moving backward from an initial position adjacent to the clamp 25.
As illustrated in FIG. 12, the driven member 102 continuously follows the cam 100 while further being lifted up as the clamp 25 opens more broader. The clamp cover 66 slightly descends while moving further backward from the clamp 25.
As illustrated in FIG. 13, the driven member 102 reaches the topmost position of the cam 100. The clamp 25 opens broadest. The clamp cover 66 descends still further.
As illustrated in FIG. 14, when the driven member 102 reaches a recess portion of the cam 100, the axis 62 begins to rotate in the other direction wherein the clamp 25 closes. Simultaneously with this movement, the clamp cover 66 switches from descending to ascending.
As illustrated in FIG. 15, when the driven member 102 follows the recess portion of the cam 100, the clamp 25 further closes and the clamp cover 66 moves back to the clamp 25 while ascending. And, the clamp 25 and the clamp cover 66 return to the initial position as shown in FIG. 10.
In the above-explained operation of the stencil printing machine proposed in Japanese Patent Application 9-183113, a front end of the clamp cover 66 describes a line (A) as shown in FIG. 17 when the clamp cover 66 opens and closes. The clamp cover 66 is connected to the other end of the arm 70 rotating around the axis 68, and the movement of the other end of the arm 70 is regulated by the link 78 pivotally movable around the pin 82. Accordingly, the clamp cover 66 describes a curved line both in opening and closing, and the curved lines in opening and closing coincide with each other.
In the constitution utilizing such link-mechanism, when the clamp 66 opens and closes, it makes large impulsive sound, which has been desired to be reduced.
An object of the present invention is to provide a stencil printing machine capable of surely preventing the ink-leakage and reducing the impulsive sound when the clamp cover opens and closes.